The question is asking about which generator in wind turbines uses slip rings and a partially rated converter for variable-speed operation. Let us analyze the options one by one to understand the correct answer.

1. Permanent Magnet Synchronous Generator (PMSG): This type of generator does not use slip rings. It is directly connected to the grid through a full-scale converter and is known for its high efficiency and compact size. Therefore, it is not the correct answer.
2. Brushless DC Generator: This generator does not use slip rings either; it uses electronic commutation instead of mechanical commutation. Hence, it is not suitable for variable-speed operation using slip rings.
3. Squirrel Cage Induction Generator (SCIG): It is typically used in fixed-speed applications because it does not use slip rings. It is connected directly to the grid and lacks flexibility for variable-speed operation.
4. Doubly-Fed Induction Generator (DFIG): This is the correct answer. The DFIG uses slip rings to allow an external rotor circuit to be connected through a partially rated converter, which allows for variable-speed operation. This setup is cost-effective because the converter only needs to manage a fraction of the total power, making it ideal for large wind turbines.

In conclusion, the doubly-fed induction generator (DFIG) uses slip rings and a partially rated converter, enabling variable-speed operation in wind turbines. This technology is advantageous because it can efficiently adapt to varying wind speeds, producing electricity more consistently and reliably.